Why Email Open Rates are Unreliable

Email marketers love to measure the effectiveness of their campaigns. From unique Emails sent to bounce-backs to click-thrus, metrics form the backbone of evaluating the success of your marketing campaign.

One metric, generally considered to be very important, but actually is extremely suspect, is the metric called “Open Rate.” The rate’s goal is to track how many people opened your Email. Why it is important is obvious. Opening your Email is like opening a letter. It indicates that some or part of the message will be read and points that the reader is at least somewhat interested in what you have to say.

Alas, a fundamental error in the way Emails are delivered prevents the open rate from being accurately measured. Because Internet Email was not designed to track open rates, Email programmers had to resort to a “hack” for this metric. In most cases, the open rates of your Emails rely on a small transparent image that is hidden from the message. The image, however is not downloaded until the reader brings in the images of the message, usually by clicking on a “Display Images” (or such) button on the Email client. When this button is clicked, the transparent image (with all the other images) is invoked from a server, and the system records that the message has been read. Typically there is a token associated with the image request, so the server knows the particular person who requested the image – i.e. who read the Email.

As you can see, this is a pretty round-about way to measure a critical statistic. In particular it suffers from the obvious flaw that the person may open and read your message without downloading images. Consequently, this has the implication that almost always the open rate that the system records is lower (sometimes far lower) than reality. In short, the number of people who open your message is usually greater than the people who download images and thus trigger an “open” read.

So you shouldn’t put a great deal of faith in your open rates. However, there are some things you can do with images to make your open rate metric closer to reality. See our article Use Images in Email to Increase Online Registration! for how to make the open rate a little bit more accurate.